Spread spectrum communications using a reference and a message signal system and method

ABSTRACT

A spread spectrum remote unit produces a message signal having message data. A reference signal is produced. The reference signal is used to recover the message data from a combined spread spectrum signal. The reference signal and the message signal is combined as a combined signal. The combined signal is transmitted as the combined spread spectrum signal.

This patent is a continuation application of U.S. patent applicationSer. No. 09/395,626, filed on Sep. 14, 1999, now U.S. Pat. No. 6,396,824which is a continuation application of U.S. patent application Ser. No.08/871,479, filed on Jun. 9, 1997, now U.S. Pat. No. 5,974,039, issuedOct. 26, 1999, which is a continuation application of U.S. patentapplication Ser. No. 08/628,012, filed on Apr. 4, 1996, now U.S. Pat.No. 5,663,956, issued Sep. 2, 1997, which is a continuation applicationof U.S. patent application Ser. No. 08/311,773, filed Sep. 23, 1994, nowU.S. Pat. No. 5,506,864, issued Apr. 9, 1996, which is a continuation ofU.S. patent application Ser. No. 08/178,016, filed Feb. 23, 1994, nowU.S. Pat. No. 5,365,544, issued Nov. 15, 1994, which was a file wrappercontinuation application of U.S. patent application Ser. No. 08/006,851,filed Jan. 21, 1993, now abandoned, which was a continuation-in-partapplication of U.S. patent application Ser. No. 07/622,235, filed Dec.5, 1990, now U.S. Pat. No. 5,351,269, issued Sep. 27, 1994, and of U.S.patent application Ser. No. 07/626,109, filed Dec. 14, 1990, now U.S.Pat. No. 5,228,056, issued Jul. 13, 1993.

BACKGROUND

This invention relates to spread-spectrum communications and moreparticularly to a system and method for locating within a cell, a remoteunit communicating synchronously with a spread-spectrum-communicationssignal using a reference carrier signal supplied on a spread-spectrumchannel by the transmitter.

DESCRIPTION OF THE RELEVANT ART

Referring to FIG. 1, message data, d(t), are processed byspread-spectrum modulator 51, using a message-chip-code signal, g₁, (t),to generate a spread-spectrum data signal. The spread-spectrum datasignal is processed by transmitter 52 using a carrier signal at acarrier frequency f_(o), and transmitted over communications channel 53.

At a receiver, a spread-spectrum demodulator 54 despreads the receivedspread-spectrum signal, and the message data are recovered bysynchronous data demodulator 60 as received data. The synchronous datademodulator 60 uses a reference signal for synchronously demodulatingthe despread spread-spectrum signal. The square-law device 55, bandpassfilter 56 and frequency divider 57 are well known in the art forgenerating a reference signal from a received modulated data signal. ACostas Loop or other reference signal generating circuit is adequate forthis purpose.

In a fading channel, such as the ionosphere or any channel containingmultipath, or more generally, any channel in which the received signal'samplitude fluctuates with time, synchronous demodulation is notpractical since the phase of the incoming signal typically is not thesame as the phase of the reference. In such cases differential phaseshift keying (DPSK) is employed. With DPSK the received signal isdelayed by one symbol and multiplied by the undelayed signal. If theresulting phase is less than ±90° a 0-bit is declared, otherwise a 1-bitis declared. Such a system is complex and suffers degradation of about 6dB at error rates of 10⁻².

The prior art does not provide a system and method for synchronouslycommunicating, using spread-spectrum modulation, with a base station andin combination locating a remote unit within the cell of a base station.

SUMMARY

A spread spectrum communication system produces a reference signal. Amessage signal is produced compromising message data. The referencesignal and the message signal is combined as a combined signal. Thecombined signal is transmitted as a combined spread spectrum signal. Thecombine spread spectrum signal is received. The reference signal isdetected within the received combined spread spectrum signal. Themessage data is recovered using the detected reference signal.

BRIEF DESCRIPTION OF THE DRAWING(S)

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a prior art scheme for synchronously recovering message data;

FIG. 2 shows a synchronous spread-spectrum system with a bitsynchronizer, synchronized to a generic chip code generator according tothe present invention;

FIG. 3A shows a synchronous spread spectrum transmitter system for aplurality of message data;

FIG. 3B shows a spread spectrum receiver using a synchronous detectorfor receiving a plurality of spread-spectrum processed signals;

FIG. 3C shows a spread spectrum receiver using a nonsynchronous detectorfor receiving a plurality of spread-spectrum processed signals;

FIG. 4 shows a synchronous spread-spectrum demodulating method;

FIG. 5 is a block diagram of a base station for communicatingsynchronously with, and geolocating, a remote unit; and

FIG. 6 is a block diagram of a remote unit for communicating with a basestation and for geolocation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Reference is now made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals indicate like elementsthroughout the several views.

The spread-spectrum communications and geolocation system and method ofthe present invention is an extension of an invention disclosed in aU.S. patent application entitled, SYNCHRONOUS-SPREAD-SPECTRUMCOMMUNICATIONS SYSTEM AND METHOD, by Donald L. Schilling, having U.S.patent application Ser. No. 07/626,109 and filing date of Dec. 14, 1990,now issued U.S. Pat. No. 5,228,056. For completeness of disclosure, thefollowing discussion includes the disclosure in the original patentapplication, and subsequently goes into a discussion for geolocation.

The spread spectrum signals of the present invention are designed to be“transparent” to other users, i.e., spread spectrum signals are designedto provide negligible interference to the communication of otherexisting users. The presence of a spread spectrum signal is difficult todetermine. This characteristic is known as low probability ofinterception (LPI) and low probability of detection (LPD). The LPI andLPD features of spread spectrum allow transmission between users of aspread spectrum CDMA communications system without the existing users ofthe mobile cellular system experiencing significant interference. Thepresent invention makes use of LPI and LPD with respect to predeterminedchannels in the mobile cellular system or in the fixed-service microwavesystem. By having the power level of each spread spectrum signal belowthe predetermined level, then the total power from all spread spectrumused within a cell does not interfere with mobile users in a mobilecellular system, or with microwave users in the fixed-service microwavesystem.

Spread spectrum is also “jam” or interference resistant. A spreadspectrum receiver spreads the spectrum of the interfering signal. Thisreduces the interference from the interfering signal so that it does notnoticeably degrade performance of the spread spectrum system. Thisfeature of interference reduction 10 makes spread spectrum useful forcommercial communications, i.e., the spread spectrum waveforms can beoverlaid on top of existing narrowband signals.

The present invention employs direct sequence spread spectrum, whichuses a phase modulation technique. Direct sequence spread spectrum takesthe power that is to be transmitted and spreads it over a very widebandwidth so that the power per unit bandwidth (watts/hertz) isminimized. When this is accomplished, the transmitted spread spectrumpower received by a mobile cellular user or a microwave user, having arelatively narrow bandwidth, is only a small fraction of the actualtransmitted power.

In a fixed-service microwave system, byway of example, if a spreadspectrum signal having a power of 10 milliwatts is spread over afixed-service microwave bandwidth of 10 MHz and a microwave user employsa communication system having a channel bandwidth of only 2 MHz, thenthe effective interfering power due to one spread spectrum signal, inthe narrow band communication system, is reduced by the factor of 10MHz/2 MHz. For fifty concurrent users of spread spectrum, the power ofthe interfering signal due to spread spectrum is increased by fifty.

The feature of spread spectrum that results in interference reduction isthat the spread spectrum receiver actually spreads the received energyof any interferer over the same wide bandwidth, 10 MHz in the presentexample, while compressing the bandwidth of the desired received signalto its original bandwidth. For example, if the original bandwidth of thedesires message data is only 30 kHz, then the power of the interferingsignal produced at a base station is reduced by 10 MHz/30 kHz.

Direct sequence spread spectrum achieves a spreading of the spectrum bymodulating the original signal with a very wideband signal relative tothe data bandwidth. This wideband signal is chosen to have two possibleamplitudes, +1 and −1, and these amplitudes are switched, in a“pseudo-random” manner, periodically. Thus, at each equally spaced timeinterval, a decision is made as to whether the wideband modulatingsignal should be +1 or −1. If a coin were tossed to make such adecision, the resulting sequence would be truly random. However, in sucha case, the receiver would not know the sequence a priori and could notproperly receive the transmission. Instead, a chip-code generatorgenerates electronically an approximately random sequence, called apseudo-random sequence, which is known a priori to the transmitter andreceiver.

Code Division Multiple Access

Code division multiple access (CDMA) is a direct sequence spreadspectrum system in which a number, at least two, of spread-spectrumsignals communicate simultaneously, each operating over the samefrequency band. In a CDMA system, each user is given a distinct chipcode. This chip code identifies the user. For example, if a first userhas a first chip code, g₁ (t), and a second user has a second chip code,g₂ (t), etc., then a receiver, desiring to listen to the first user,receives at its antenna all of the energy sent by all of the users.However, after despreading the first user's signal, the receiver outputsall the energy of the first user but only a small fraction of theenergies sent by the second, third, etc., users.

CDMA is interference limited. That is, the number of users that can usethe same spectrum and still have acceptable performance is determined bythe total interference power that all of the users, taken as a whole,generate in the receiver. Unless one takes great care in power control,those CDMA transmitters which are close to the receiver causeoverwhelming interference. This effect is known as the “near-far”problem. In a mobile environment the near-far problem could be thedominant effect. Controlling the power of each individual mobile remoteuser is possible so that the received power from each mobile remote useris the same. This technique is called “adaptive power control”. See U.S.Pat. No. 5,093,840, having issue date of Mar. 3, 1992, entitled,ADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUM SYSTEM AND METHOD, byDonald L. Schilling, which is incorporated herein by reference.

The spread spectrum communications system of the present invention is acode division multiple access (CDMA) system. Spread spectrum CDMA cansignificantly increase the use of spectrum. With CDMA, each user in acell uses the same frequency band. However, each CDMA signal has aseparate pseudo random code which enables a receiver to distinguish adesired signal from the remaining signals. Remote users in adjacentcells use the same frequency band and the same bandwidth, and therefore“interfere” with one another. A received signal may appear somewhatnoisier as the number of users' signals received by a PCN base stationincreases.

Each unwanted user's signal generates some interfering power whosemagnitude depends on the processing gain. Remote users in adjacent cellsincrease the expected interfering energy compared to remote users withina particular cell by about 50%, assuming that the remote users areuniformly distributed throughout the adjacent cells. Since theinterference increase factor is not severe, frequency reuse is notemployed.

Each spread spectrum cell can use a full 10 MHz band for transmissionand a fall 10 MHz band for reception. Hence, using a chip rate of fivemillion chips per second and a coding data rate of 4800 bps results inapproximately a processing gain of 1000 chips per bit. It is well knownto those skilled in the art: that the maximum number of CDMA remoteusers that can concurrently use a frequency band is approximately equalto the processing gain.

Synchronous Spread Spectrum Communications

As illustratively shown in FIG. 2, a spread spectrum code divisionmultiple access (CDMA) communications system for use over acommunications channel 110 is provided comprising generic means, messagemeans, spreading means, summer means, transmitting means,generic-spread-spectrum-processing means,message-spread-spectrum-processing means, acquisition and trackingmeans, detection means and synchronous means. The generic means andmessage means are embodied as a transmitter-generic-chip-code generator101 and transmitter-message-chip-code generator 102. The spreading meansis shown as an EXCLUSIVE-OR device 103, which may be an EXCLUSIVE-ORgate. Summer means is a combiner 105 and the transmitting means includesa transmitter which is embodied as a signal source 108 coupled tomodulator 107. The transmitter-message-chip-code generator 102 iscoupled to the EXCLUSIVE-OR device 103. Thetransmitter-generic-chip-code generator 101 is shown coupled to thetransmitter-message-chip-code generator 102 and the source for messagedata. The EXCLUSIVE-OR device 103 and the transmitter-generic-chip-codegenerator 101 are coupled to the combiner 105. The modulator 107 iscoupled between the combiner 105 and the communications channel 110.

At the receiver the generic-spread-spectrum-processing means is embodiedas the receiver-generic-chip-code generator 121, the generic mixer 123and the generic-bandpass filter 125. The generic mixer 123 is coupledbetween the receiver-generic-chip-code generator 121 and thegeneric-bandpass filter 125. The message-spread-spectrum-processingmeans is embodied as a receiver-message-chip-code generator 122, amessage mixer 124 and, a message-bandpass filter 126. The message mixer124 is coupled between the receiver-message-chip-code generator 122 andthe message-bandpass filter 126. A power splitter 115 is coupled betweenthe communications channel 110, and the generic mixer 123 and themessage mixer 124.

The acquisition and tracking means is embodied as an acquisition andtracking circuit 131. The acquisition and tracking circuit 131 iscoupled to an output of the generic-bandpass filter 125, and to thereceiver-generic-chip-code generator 121. The receiver-message-chip-codegenerator 122 preferably is coupled to the receiver-generic-chip-codegenerator 121.

The detection means is embodied as a detector 139. The detector 139 iscoupled to the message-bandpass filter 126 and the generic-bandpassfilter 125. The detector 139 may be a nonsynchronous detector such as anenvelope detector or square-law detector. Alternatively, the detector139 may be a synchronous detector, which uses a recovered-carrier signalfrom the generic-bandpass filter 125.

The synchronous means includes bit means, a lowpass filter 128 andelectronic switch 130. The bit means is embodied as a bit synchronizer129. The lowpass filter 128 and electronic switch 130 are coupled to thebit synchronizer 129. The bit synchronizer 129, as shown in FIG. 2,preferably is coupled to the receiver-generic-chip-code generator 121.Alternatively, the bit synchronizer 129 may be coupled to an output ofthe detector 139.

The transmitter-generic-chip-code generator 101 generates ageneric-chip-code signal, g₀ (t), and the transmitter-message-chip-codegenerator 102 generates a message-chip-code signal, g₁ (t). Synchronoustiming of the message data, d₁ (t), and the message-chip-code signal, inFIG. 2, is provided by the generic-chip-code signal, although othersources can be used such as a common clock signal for synchronization.The EXCLUSIVE-OR device 103 generates a spread-spectrum signal byspread-spectrum processing message data with the message-chip-codesignal. The spread-spectrum processing may be accomplished by modulo-2adding the message data to the message-chip-code signal. The combiner105 combines the generic-chip-code signal with thespread-spectrum-processed signal. The combined generic-chip-code signal.and spread-spectrum-processed signal may be a multilevel signal, havingthe instantaneous voltage levels of the generic-chip-code signal and thespread-spectrum-processed signal.

The modulator 107, as part of the transmitter, modulates the combinedgeneric-chip-code signal and spread-spectrum-processed signal by acarrier signal, cos ω_(o) t, at a carrier frequency, f_(o). Themodulated generic-chip-code signal and spread-spectrum processed signalare transmitted over the communications channel 110 as a code divisionmultiple access (CDMA) signal, x_(c) (t). Thus, the CDMA signal includesthe generic-chip-code signal and the spread-spectrum-processed signal asif they were each modulated separately, and synchronously, on separatecarrier signals having the same carrier frequency, f_(o), andtransmitted over the communications channel.

At a receiver, the generic-spread-spectrum-processing means recovers thecarrier signal, cos ω_(o) t, from the CDMA signal, x_(c) (t), and themessage-spread-spectrum-processing means despreads the CDMA signal,x_(c) (t), as a modulated-data signal, d₁ (t). More particularly,referring to FIG. 2, the CDMA signal received from the communicationschannel 110, is divided by power splitter 115. Thereceiver-generic-chip-code generator 121 generates a replica of thegeneric-chip-code signal, g₀ (t). The generic mixer 123 uses the replicaof the generic-chip-code signal for despreading the CDMA signal, x_(c)(t), from the power splitter 115, as a recovered-carrier signal. Thespread-spectrum channel, of the CDMA signal having the generic-chip-codesignal, g₀ (t) cos ω_(o) t, generally does not include data so thatdespreading the CDMA signal produces the carrier signal, only. Thegeneric-bandpass filter 125 filters the recovered-carrier signal at thecarrier frequency, or equivalently, at an intermediate frequency. Incomparison to the message-bandpass filter 126 which has a bandwidthsufficiently wide for filtering a modulated-data signal, thegeneric-bandpass filter 125 can have a very narrow bandwidth forfiltering the recovered-carrier signal. The very narrow bandwidth of thegeneric-bandpass filter 125 assists in extracting the recovered-carriersignal from noise.

The acquisition and tracking circuit 131 acquires and tracks therecovered-carrier signal from an output of the generic-bandpass filter125. The replica of the generic-chip-code signal from thereceiver-generic-chip-code generator 121 is synchronized to therecovered-carrier signal via acquisition and tracking circuit 131.

The receiver-message-chip-code generator 122 generates a replica of themessage-chip-code signal, g₁ (t). The replica of the message-chip-codesignal, g₁ (t), is synchronized to the replica of the generic-chip-codesignal, g₀ (t), from the receiver-generic-chip-code generator 121. Thus,the receiver-message-chip-code generator 122, via synchronization to thereceiver-generic-chip-code generator 121, has the same synchronizationas the transmitter-message-chip-code generator 102 via synchronizationto the transmitter-generic-chip-code generator 101. Accordingly, thespread-spectrum communications channel having the generic-chip-codesignal provides coherent spread-spectrum demodulation of thespread-spectrum channels with data.

The message mixer 124 uses the replica of the message-chip-code signalfor despreading the CDMA signal from the power splitter 115, to generatea modulated-data signal, d₁ (t) cos ω_(o) t. The modulated-data signaleffectively is the message data modulated by the carrier signal. Themessage-bandpass filter 126 filters the modulated-data signal at thecarrier frequency, or equivalently at an intermediate frequency (IF).Down converters, which convert the modulated-data signal to an IF,optionally may be used without altering the cooperative functions orteachings of the present invention.

The detector 139 demodulates the modulated-data signal as a detectedsignal. The detected signal is filtered through lowpass filter 128,sampled by electronic switch 130 and outputted as received data, d₁ (t).The received data, without errors, are identical to the message data.The lowpass filter 128 and electronic switch 130 operate in an“integrate and dump” function, respectively, under the control of thebit synchronizer 129.

The bit synchronizer 129 controls the integrating and dumping of lowpassfilter 128 and electronic switch 130. The bit synchronizer 129preferably derives synchronization using the replica of thegeneric-chip-code signal from the receiver-generic-chip-code generator121 as illustrated in FIG. 2. The bit synchronizer 129 also may derivesynchronization from an output of the detector 139, as illustrated inFIG. 1.

In a preferred embodiment, the bit synchronizer 129 receive the replicaof the generic-chip-code signal, g₀ (t), from thereceiver-generic-chip-code generator 121. The replica of thegeneric-chip-code signal, by way of example, may include a chip codewordhaving 8250 chips. Assuming that there are eleven bits per chipcodeword, then there are 750 chips per bit of data. Since the replica ofthe generic-chip-code signal provides information to the bitsynchronizer 129 as to where the chip codeword begins, the bitsynchronizer 129 thereby knows the timing of the corresponding bits forsynchronization.

The present invention further may include transmitting as the CDMAsignal, a plurality of spread-spectrum-processed signals for handling aplurality of message data. In this case the invention includes aplurality of message means and a plurality of spreading means. Referringto FIG. 3A, the plurality of message means may be embodied as aplurality of transmitter-message-chip-code generators and the pluralityof spreading means may be embodied as a plurality of EXCLUSIVE-OR gates.The plurality of transmitter-message-chip-code generators generates aplurality of message-chip-code signals. In FIG. 3A, the plurality oftransmitter-message-chip-code generators is shown as firsttransmitter-message-chip-code generator 102 generating firstmessage-chip-code signal, g₁ (t), second transmitter-message-chip-codegenerator 172 generating second message-chip-code signal, g₂ (t),through N^(th) transmitter-message-chip-code generator 182 generatingN^(th) message-chip-code signal, g_(N) (t). The plurality ofEXCLUSIVE-OR gates is shown as first EXCLUSIVE-OR gate 103, secondEXCLUSIVE-OR gate 173, through N^(th) EXCLUSIVE-OR gate 183. Theplurality of EXCLUSIVE-OR gates generates a plurality ofspread-spectrum-processed signals by modulo-2 adding the plurality ofmessage data d₁ (t), d₂ (t), . . . , d_(N) (t) with the plurality ofmessage-chip-code signals g₁ (t), g₂ (t), . . . , g_(N) (t),respectively. More particularly, the first message data, d₁ (t), aremodulo-2 added with the first message-chip-code signal, g₁ (t), thesecond message data, d₂ (t), are modulo-2 added with the secondmessage-chip-code signal, g₂ (t), through the N^(th) message data, d_(N)(t), which are modulo-2 added with the N^(th) message-chip-code signal,g_(N) (t).

The transmitter-generic-chip-code generator 101 is coupled to theplurality of transmitter-message-chip-code generators and the source forthe plurality of message data, d₁ (t), d₂ (t), . . . d_(N) (t). Thegeneric-chip-code signal g₀ (t), in a preferred embodiment, providessynchronous timing for the plurality of message-chip-code signals g₁(t), g₂ (t), . . . , g_(N) (t), and the plurality of message data d₁(t), d₂ (t), . . . , d_(N) (t).

The combiner 105 combines the generic-chip-code signal and the pluralityof spread-spectrum-processed signals, by linearly adding thegeneric-chip-code signal with the plurality of spread-spectrum-processedsignals. The combined signal typically is a multilevel signal, which hasthe instantaneous voltage levels of the generic-chip-code signal and theplurality of spread-spectrum-processed signals.

The modulator 107, as part of the transmitter, modulates the combinedgeneric-chip-code signal and the plurality of spread-spectrum-processedsignals by a carrier signal, cos ω_(o) t, at a carrier frequency, f_(o).The modulated generic-chip-code signal and the plurality ofspread-spectrum processed signals are transmitted over thecommunications channel 110 as a CDMA signal, x_(c) (t). The CDMA signal,x_(c) (t) has the form:

${x_{c}(t)} = {{g_{0}(t)} + {\sum\limits_{1}^{N}{\left\lbrack {{g_{i}(t)} + {d_{i}(t)}} \right\rbrack\cos\;\omega_{0}t}}}$Thus, the CDMA signal includes the generic-chip-code signal and theplurality of spread-spectrum-processed signals as if they were eachmodulated separately, and synchronously, on separate carrier signalswith the same carrier frequency, f_(o), and transmitted over thecommunications channel.

The present invention includes receiving a CDMA signal which has aplurality of spread-spectrum-processed signals. The receiver furtherincludes a plurality of message-spread-spectrum processing means, aplurality of detection means and a plurality of synchronous means. Theplurality of message-spread-spectrum-processing means, as shown in FIG.3B, may be embodied as a plurality of message-chip-code generators, aplurality of message mixers and a plurality of message-bandpass filters.A mixer is connected between a respective message-chip-code generatorand message-bandpass filter. The plurality of message mixers is coupledto the power splitter 115. More particularly, the plurality ofmessage-chip-code generators is shown embodied as firstmessage-chip-code generator 122, second message-chip-code generator 172,through N^(th) message-chip-code generator 182. The plurality of messagemixers is shown as first message mixer 124, second message mixer 174through N^(th) message mixer 184. The plurality of message-bandpassfilters is shown as first message-bandpass filter 126, secondmessage-bandpass filter 176, through N^(th) message-bandpass filter 186.

The plurality of detection means may be embodied as a plurality ofsynchronous detectors which is shown as first synchronous detector 127,second synchronous detector 177 through N^(th) synchronous detector 187.Each of the plurality of synchronous detectors are coupled to one of theplurality message-bandpass filters.

The plurality of synchronous means may include a bit synchronizer 129, aplurality of lowpass filters and a plurality of electronic switches. Theplurality of lowpass filters is shown as first lowpass filter 128,second lowpass filter 178, through N^(th) lowpass filter 188. Theplurality of electronic switches is shown as first electronic switch130, second electronic switch 180 through N^(th) electronic switch 190.Each of the plurality of synchronous detectors is coupled to an outputof the generic-bandpass filter 125. The recovered-carrier signal fromthe generic-bandpass filter 125 serves as the reference signal forsynchronously demodulating each of the plurality of message-data signalsby the plurality of synchronous detectors, as a plurality of receiveddata, d₁ (t), d₂ (t), . . . , d_(N) (t).

The detection means alternatively may be embodied as a plurality ofnonsynchronous detectors, such as envelope detectors 139, 189, 199, asshown in FIG. 3C. Typically, the nonsynchronous detectors do not requirethe recovered-carrier signal.

The bit synchronizer 129 derives timing from the replica of thegeneric-chip-code signal, g₀ (t), and controls the timing of theintegrating and dumping functions of the plurality lowpass filters andthe plurality of electronic switches.

With the use of the invention as embodied in FIG. 3B, ageneric-spread-spectrum channel, as part of the CDMA signal, providesthe recovered-carrier signal, as discussed previously. The acquisitionand tracking circuit 131 acquires and tracks the recovered-carriersignal from an output of the generic-bandpass filter 125. The replica ofthe generic-chip-code signal from the receiver-generic-chip-codegenerator 121 is synchronized to the recovered-carrier signal viaacquisition and tracking circuit 131. The receiver-generic-chip-codegenerator 121 generates a replica of the generic-chip-code signal, g₀(t), which provides timing to bit synchronizer 129 and to the pluralityof receiver-message-chip-code generators 122, 172, 182.

The present invention also includes a method for synchronouslydemodulating a CDMA signal. Message data are input to the spreadingmeans. Referring to FIG. 4, the method comprises the steps of generating403 a generic-chip-code signal. The method further includes generating405 message data synchronized to the generic-chip-code signal, andgenerating 407 a message-chip-code signal synchronized 408 to thegeneric-chip-code signal. Message data are processed, using aspread-spectrum modulator, with the message-chip-code signal to generatea spread-spectrum-processed signal. The generic-chip-code signal iscombined 409 with the spread-spectrum-processed signal. The methodtransmits 411 the combined generic-chip-code signal andspread-spectrum-processed signal on a carrier signal over thecommunications channel as a CDMA signal, which the receiver receives 412as a spread spectrum communications signal.

At a receiver, the method includes recovering 413 the carrier signalfrom the CDMA signal and despreading 415 the CDMA signal as amodulated-data signal. The recovered-carrier signal is used tosynchronize the step of despreading the CDMA signal and to optionallysynchronously demodulate 417 and output 419 the modulated-data signal asreceived data.

In use of system as set forth in FIG. 3A, thetransmitter-generic-chip-code generator 101 generates thegeneric-chip-code signal, g₀ (t). Message data are spread-spectrumprocessed by the EXCLUSIVE-OR device 103 with message-chip-code signal,g₁ (t), from transmitter-message-chip-code generator 102. The combiner105 combines the generic-chip-code signal with thespread-spectrum-processed signal. The combined signal may be, forexample, a multilevel signal, which is generated by linearly adding thevoltage levels of the generic-chip-code signal and thespread-spectrum-processed signal, or by adding the voltage levels of thegeneric-chip-code signal with a plurality of spread-spectrum-processedsignals. The transmitter transmits on a carrier signal having a carrierfrequency, f_(o), the combined generic-chip-code signal and theplurality of spread-spectrum-processed signals. The CDMA signal istransmitted through the communications channel 110.

At the receiver, as shown in FIG. 3B, thegeneric-spread-spectrum-processing means, embodied as thereceiver-generic-chip-code generator 121, the generic mixer 123 and thegeneric-bandpass filter 125, cooperatively operate to recover thecarrier signal from the CDMA signal. Themessage-spread-spectrum-processing means, embodied as thereceiver-message-chip-code generator 122, the message mixer 124 and themessage-bandpass filter 126, cooperatively despread the CDMA signal asthe modulated-data signal. The receiver-message-chip-code generator 122preferably is synchronized to the replica of the generic-chip-codesignal from the receiver-generic-chip-code generator 121. A plurality ofreceiver-message-chip-code generators may be employed, synchronized tothe replica of the generic-chip-code signal. The synchronous means,embodied as the synchronous detector 127 synchronized to therecovered-carrier signal, demodulates the modulated-data signal asreceived data.

The received data are integrated and dumped by lowpass filter 128 andelectronic switch 130, under control of the bit synchronizer 129. Thebit synchronizer 129 preferably uses the replica of thegeneric-chip-code signal for synchronizing the integrate and dumpfunctions.

Spread Spectrum Geolocation

A spread spectrum code division multiple access (CDMA) communicationsand geolocation system and method for use over a communications channelis provided comprising at least one base station and a plurality ofremote units. The remote units may be mobile or in a fixed, stationarylocation. Message data are communicated between the base stations andthe remote units. Message data include, but are not limited to,digitized voice, computer data, facsimile data, video data, etc. Thebase station communicates base-message data to the plurality of remoteunits. A remote unit communicates remote-message data to the basestation. Base-message data are defined herein to be message dataoriginating from a base station, and remote-message data are definedherein to be message data originating from a remote unit. The followingdiscussion is of a preferred embodiment with the range between the basestation and remote unit being determined at the base station. The rolesof the base station and remote unit can be interchanged, as anequivalent to those skilled in the art, with the range being determinedat the remote unit.

In the exemplary arrangement shown in FIG. 5, a base station includesbase-spreading means, base-generic means, base-combiner means,base-transmitter means, and base antenna. The term “base” is used as aprefix to indicate an element is located at the base station, or that asignal originates from a base station.

The base-spreading means spread-spectrum processes the base-messagedata, d₁ (t). The base-spreading means is embodied as abase-spread-spectrum modulator. The base-spread-spectrum in modulator isshown as a message-chip-code generator 502 and an EXCLUSIVE-OR gate 503.The EXCLUSIVE-OR gate 503 is coupled to the message-chip-code generator502. The message-chip-code generator 502 uses a chip codeword forgenerating a chip-code sequence for spread-spectrum processingbase-message data, d₁ (t). The chip-code sequence from message-chip-codegenerator 502 is spread-spectrum processed by modulo addition byEXCLUSIVE-OR gate 503. Many equivalent circuits can be used for thebase-spread-spectrum modulator, including but not limited to, productdevices for multiplying the chip-code sequence by the base-message data,matched filters and surface acoustic wave devices which have an impulseresponse matched to the chip-code sequence, as is well known to thoseskilled in the art.

The base-generic means generates a base-generic-chip-code signal. Theterm “generic” is used as a prefix to indicate that thegeneric-chip-code signal is an unmodulated, or low data rate,direct-sequence spread-spectrum signal, which can serve as a pilotchannel. The pilot channel allows a user to acquire timing, and providesa phase reference for coherent demodulation. The base-generic means isembodied as a base-generic-chip-code generator 501. Thebase-generic-chip-code generator 501 generates a base-generic-chip-codesignal, using a chip codeword commonly shared with all remote unitscommunicating with the base station. The message-chip-code generator 502is coupled to the base-generic-chip-code generator 501, for derivingcommon timing. Alternatively, a common clock can be used for providingthe timing signal to the message-chip-code generator 502 and thebase-generic-chip-code generator 501.

The base-combiner means combines the base-generic-chip-code signal withthe spread-spectrum-processed-base-message data, to generate a base-CDMAsignal. The base-combiner means is embodied as a base combiner 505. Thebase combiner 505 is coupled to the base-generic-chip-code generator 501and the EXCLUSIVE-OR gate 503. The base combiner 505 linearly adds thebase-generic-chip-code signal with thespread-spectrum-processed-base-message data from EXCLUSIVE-OR gate 503.The resulting signal at the output of the base combiner 505 is a codedivision multiple access (CDMA) signal, denoted herein as the base-CDMAsignal. Selected variations of nonlinear combining also may be used, solong as the resulting base-CDMA signal can have its channels detected ata spread-spectrum receiver.

The base-transmitter means transmits the base-CDMA signal from the basestation to a remote unit. The base-transmitter means is embodied as asignal source 508 and product device 507. The product device 507 iscoupled between the base combiner 505 and the signal source 508. Thesignal source 508 generates a first carrier signal at a first carrierfrequency f₁. The base-CDMA signal, from the output of the base combiner505, is multiplied by the first carrier signal by product device 507.Other transmitting devices are well known in the art for putting adesired signal at a selected carrier frequency.

The base antenna 509 is coupled through an isolator 513 to thebase-transmitter means. The base antenna 509 radiates the base-CDMAsignal at the first carrier frequency.

As illustratively shown in FIG. 6, a remote unit includes a remoteantenna 511, remote-detection means, remote-spreading means,remote-combiner means, and remote-transmitter means. Each remote unitalso may include remote-generic means. The term “remote” is used as aprefix to indicate an element is located at a remote unit, or that asignal originates from the remote unit. The remote antenna 511 receivesthe base-CDMA signal radiated from the base station.

The remote-detection means is coupled to the remote antenna 511. Theremote-detection means detects the base-generic-chip-code signalembedded in the base-CDMA signal. Using thedetected-base-generic-chip-code signal, the remote-detection meansrecovers the base-message data communicated from the base station. Aremote unit can retransmit the detected-base-generic-chip-code signal,or optionally, can have remote-generic means generate a differentremote-generic-chip-code signal.

In FIG. 6, the remote-detection means is embodied as a product device536, bandpass filter 537, acquisition and tracking circuit 538,generic-chip-code generator 539, message-chip-code generator 541,product device 542, bandpass filter 543, data detector 544, lowpassfilter 545, and bit synchronizer 540. As is well known in the art, otherdevices and circuits can be used for the same function, including butnot limited to, matched filters, surface acoustic wave devices, etc.This circuit acquires and tracks the base-generic-chip-code signalembedded in the base-CDMA signal. The base-CDMA signal is received atremote antenna 511, and passes through isolator 534 and power splitter535. The base-generic-chip-code signal is detected using product device536, bandpass filter 537, acquisition and tracking circuit 538 andgeneric-chip-code generator 539. The function of this circuit is asdescribed in the previous section. The detected-base-generic-chip-codesignal is used to recover the base-messages data embedded in thebase-CDMA signal, using message-chip-code generator 541, product device542, bandpass filter 543, data detector 544, lowpass filter 545, and bitsynchronizer 540. The data detector 544 may operate coherently ornoncoherently. The detected base-message data is outputted as detecteddata, d_(R1) (t).

If the base-generic-chip-code signal is to be combined as part of theremote-CDMA signal, then generic-chip-code generator 546 is notrequired, since the base-generic-chip-code signal is available at theoutput of the generic-chip-code generator 539. If aremote-generic-chip-code signal, which is different from thebase-generic-chip-code signal, is to be used, then the generic-chip-codegenerator 546 can be used for generating the remote-generic-chip-codesignal. In the latter case, the remote-generic-chip-code signal isclocked or synchronized with the detected base-generic-chip-code signal.For purposes of discussion, the remote-generic-chip-code signal isconsidered to be sent from the remote unit to the base station, with theunderstanding that the remote-generic-chip-code signal can be identicalto, or one and the same as, the detected base-generic-chip-code signal.

The remote-spreading means spread-spectrum processes remote-messagedata. The remote-spreading means is embodied as a remote-spread-spectrummodulator. The remote-spread-spectrum modulator is shown as amessage-chip-code generator 548 and an EXCLUSIVE-OR gate 547. TheEXCLUSIVE-OR gate 547 is coupled to the message-chip-code generator 548.The message-chip-code generator 548 uses a chip codeword for generatinga chip-code sequence for spread-spectrum processing remote-message data,d₂ (t). The chip-code sequence from message-chip-code generator 548 isspread-spectrum processed by modulo addition by EXCLUSIVE-OR gate 547.Many equivalent circuits can be used for the remote-spreading means,including but not limited to, product devices for multiplying thechip-code sequence by the base-message data, matched filters and surfaceacoustic wave devices, as is well known to those skilled in the art.

The remote-generic-chip-code signal and thespread-spectrum-processed-remote-message data are combined by theremote-combiner means, as a remote-CDMA signal. The remote-combinermeans is embodied as a remote-combiner 549. The remote combiner 549 iscoupled to the EXCLUSIVE-OR gate 547, and the remote-generic-chip-codegenerator 546, or alternatively to the generic-chip-code generator 539.The remote combiner 549 linearly adds the remote-generic-chip-codesignal with the spread-spectrum-processed-remote-message data fromEXCLUSIVE-OR gate 547. The resulting signal at the output of the remotecombiner 549 is a code division multiple access (CDMA) signal, denotedherein as the remote-CDMA signal. Selected variations of nonlinearcombining also may be used, so long as the resulting remote-CDMA signalcan have its channels detected at a spread-spectrum receiver.

The remote unit also includes the remote-transmitter means fortransmitting the remote-CDMA signal from the remote unit to the basestation. The remote-transmitter means is embodied as a signal source 551and product device 550. The product device 550 is coupled between theremote combiner 549 and the signal source 551. The signal source 551generates a carrier signal at a second carrier frequency f₂. Theremote-CDMA signal, from the output of the remote combiner 549, ismultiplied by the second carrier signal by product device 550. Othertransmitting devices are well known in the art for putting a desiredsignal at a selected carrier frequency. The second carrier frequency maybe the same as, or different from, the first carrier frequency.

The remote antenna 511 is coupled through an isolator 534 to theremote-transmitter means. The remote antenna 511 radiates theremote-CDMA signal at the second carrier frequency.

Each of the base stations further includes base-detection means andrange means. The base-detection means is coupled to the base antenna 509through isolator 513 and power splitter 515. The base detection meansdetects the remote-generic-chip-code signal embedded in the remote-CDMAsignal. The base-detection means, as illustrated in FIG. 5, may beembodied as a base detector which may includes a product device 523,bandpass filter 525, acquisition and tracking circuit 531,generic-chip-code generator 521, message-chip-code generator 522,product device 524, bandpass filter 526, data detector 527, lowpassfilter 528, and bit synchronizer 529. As is well known in the art, thebase detection means may be embodied with other devices and circuitswhich perform the same function, including but not limited to, matchedfilters, surface acoustic wave devices, etc. This circuit acquires andtracks the remote-generic-chip-code signal embedded in the remote-CDMAsignal. The remote-CDMA signal is received at base antenna 509, andpasses through isolator 513 and power splitter 515. Theremote-generic-chip-code signal is detected using product device 523,bandpass filter 525, acquisition and tracking circuit 531 andgeneric-chip-code generator 521. The function of this circuit is aspreviously described. The detected-remote-generic-chip-code signal isused to recover the remote-message data embedded in the remote-CDMAsignal, using message-chip-code generator 522, product device 524,bandpass filter 526, data detector 527, lowpass filter 528, and bitsynchronizer 529. The data detector 527 may operate coherently ornoncoherently. The detected remote-message data is outputted as detecteddata, dR2 (t). Thus, the base detector recovers, using thedetected-remote-generic-chip-code signal, the remote message datacommunicated from the remote unit.

Using the detected-remote-generic-chip-code signal and thebase-generic-chip-code signal, the range means determines a range delaybetween the remote unit and the base station. The range means isembodied as a range delay device 530, which can compare the timingbetween the base-generic-chip-code signal from the generic-chip-codegenerator 501, with the detected remote-generic-chip-code signal fromthe generic-chip-code generator 521.

The present invention may include further the steps of spread-spectrumprocessing the base-message data; generating a base-generic-chip-codesignal; combining the base-generic-chip-code signal with thespread-spectrum-processed-base-message data, thereby generating abase-CDMA signal; transmitting the base-CDMA signal from the basestation to the remote unit; detecting the base-generic-chip-code signalembedded in the base-CDMA signal; recovering, using thedetected-base-generic-chip-code signal, the base-message data;spread-spectrum processing remote-message data; generating, using thedetected-generic-chip-code signal and thespread-spectrum-processed-remote data, a remote-CDMA signal;transmitting the remote-CDMA signal from the remote unit to the basestation; detecting the remote-generic-chip-code signal embedded in theremote-CDMA signal; recovering, using thedetected-remote-generic-chip-code signal, the remote-message data; anddetermining, using the detected-remote-generic-chip-code signal and thebase-generic-chip-code signal, a range delay between the remote unit andthe base station.

In use, the base station spread-spectrum processes the base-message datawith a message-chip-code signal, and combines thespread-spectrum-processed-base-message data with abase-generic-chip-code signal. The combined signal is a base-CDMA signalwhich is transmitted over a communications channel to at least oneremote unit.

The remote unit receives the base-CDMA signal, detects thebase-generic-chip-code signal embedded in the base-CDMA signal, and usesthe detected-base-generic-chip-code signal to recover the base-messagedata embedded in the base-CDMA signal.

The detected base-generic-chip-code signal is relayed as aremote-generic-chip-code signal, or is used to set the timing for adifferent remote-generic-chip-code signal, which is sent from the remoteunit to the base station. The remote unit spread-spectrum processes theremote-message data with a remote-chip-code signal, and combines thespread-spectrum-processed-remote-message data with theremote-generic-chip-code signal as a remote-CDMA signal. The remote-CDMAsignal is sent over the communications channel to the base station.

At the base station, the remote-generic-chip-code signal is detectedfrom the remote-CDMA signal, and the detected remote-generic-chip-codesignal is used to detect the remote-message data embedded in theremote-CDMA signal. Additionally, the detected remote-generic-chip-codesignal is compared with the base-generic-chip-code signal in arange-delay circuit, to determine the range of the remote unit from thebase station. Effectively, the range between the remote unit and thebase station is a function of the timing between sending a sequence ofthe chip codeword which generated the base-generic-chip-code signal, andreceiving the sequence generated by the chip codeword which generatedthe remote-generic-chip-code signal.

The concept of using a radio frequency (RF) signal to determine range iswell known in the art. The RF signal is subject to a fixed rate ofpropagation, 3×10⁸ meters/sec. The RF signal leaves a transmitter sometime before it reaches a receiver. A particular sequence of thebase-generic-chip-code signal and remote-generic-chip-code signal areused as a mark in time. The difference in time of the sequence of thebase-generic-chip-code signal as seen at the receiver of the remoteunit, from that present at the transmitter of the base station, isrelated directly to distance between the base station and remote unit.Similarly, the difference in time of the sequence of theremote-generic-chip-code signal as seen at the receiver of the basestation from that present at the transmitter of the remote unit, isrelated directly to distance between the remote unit and base station.

The use of the base-generic-chip-code signal and theremote-generic-chip-code signal is a common type of echo rangemeasurement method that is used in radar systems. Many radar systemssimply employ a pulse of RF energy and then wait for a return of aportion of the energy due to the pulse being reflected from objects. Theradar marks time from the instant of pulse transmission until itsreturn. The time required for the pulse to return is a function of thetwo-way range to the object. The range is easily determined from thesignal propagation speed.

The spread-spectrum signals of the present invention are subject to thesame distance/time relationship. The spread-spectrum signal of thepresent invention has an advantage in that its phase is easilyresolvable. The basic resolution of a sequence of a base-chip-codesignal or a remote-chip-code signal is one code chip. Thus, the higherthe chip rate, the better the measurement capability. Thus, at a chiprate of 10 Mchips/sec, a basic range resolution is 10⁻⁷ seconds, or 30meters.

Additional delays may be encountered in the circuitry of the remoteunit. These delays can be compensated at the base station, whendetermining the distance between the base station and the remote unit.

It will be apparent to those skilled in the art that variousmodifications can be made to the synchronous spread-spectrumcommunications system and method of the instant invention withoutdeparting from the scope or spirit of the invention, and it is intendedthat the present invention cover modifications and variations of thesynchronous spread-spectrum communications system and method providedthey come in the scope of the appended claims and their equivalents.

1. A spread spectrum communication system comprising: means forproducing a reference signal; means for producing a message signalhaving message data; means for combining the reference signal and themessage signal as a combined signal; means for transmitting the combinedsignal as a combined spread spectrum signal; means for receiving thecombined spread spectrum signal; means, including a plurality ofdetector means, for detecting the reference signal within the receivedcombined spread spectrum signal; and means for recovering the messagedata using information from the detected reference signal by generatinga local reference signal and synchronizing the local reference signalwith the detected reference signal, said means for recoveringdemodulating each of a plurality of message data signals.
 2. The systemof claim 1 further comprising: means for recovering a carrier signalfrom the combined spread spectrum signal using the local referencesignal; and wherein the message data recovering means uses the recoveredcarrier signal to recover the message data.
 3. The system of claim 1wherein the combined spread spectrum signal is in a code divisionmultiple access format.
 4. The system of claim 1 wherein a base stationtransmits the combined spread spectrum signal and a mobile stationreceives the combined spread spectrum signal.
 5. The communicationsystem of claim 1, wherein: the means for generating the local referencesignal relays the detected reference signal, the local reference signaltransmitted as part of the combined spread spectrum signal; and a secondunit spread-spectrum processes message data with the local referencesignal, and combines the spread-spectrum-processed message data with thelocal reference signal as a CDMA signal.
 6. The communication system ofclaim 1, wherein: the means for generating the local reference signaluses the detected reference signal to set the timing for the localreference signal, the local reference signal transmitted as part of thecombined spread spectrum signal; a second unit spread-spectrum processesthe message data with the local reference signal, and combines thespread-spectrum-processed message data with the local reference signalas a CDMA signal; and the second unit transmits said CDMA signal over acommunications channel.
 7. A method for use in a spread spectrumcommunication system, the method comprising: producing a referencesignal; producing a message signal having message data; combining thereference signal and the message signal as a combined signal;transmitting the combined signal as a combined spread spectrum signal;receiving the combined spread spectrum signal; generating a localreference signal at a second unit; synchronizing the local referencesignal with a detected reference signal at the second unit; usingdetector means and synchronization means to detect the reference signalwithin the received combined spread spectrum signal; synchronizing thelocal reference signal with the detected reference signal; andrecovering the message data of the message signal using information fromthe detected reference signal, thereby demodulating the message signal.8. The method of claim 7 further comprising: recovering a carrier signalfrom the combined spread spectrum signal; and wherein the step ofrecovering the message data uses the recovered carrier signal to recoverthe message data.
 9. The method of claim 7 wherein the combined spreadspectrum signal is in a code division multiple access format.
 10. Themethod of claim 7 wherein a base station transmits the combined spreadspectrum signal and a mobile station receives the combined spreadspectrum signal.
 11. The method of claim 7, further comprising: usingthe generation of the local reference signal at the second unit to relaythe detected reference signal, and transmitting the local referencesignal from the second unit; processing the local message data with thelocal reference signal, and combining the spread-spectrum-processedlocal message data with the local-reference signal as a local-CDMAsignal; and transmitting the local-CDMA signal over a communicationschannel.
 12. The method of claim 7, further comprising: generating thelocal reference signal at the second unit using the detected referencesignal to set the timing for the local reference signal; processing thelocal message data with the local reference signal, and combines thespread-spectrum-processed local message data with the local referencesignal as a CDMA signal; and transmitting the CDMA signal over acommunications channel.
 13. A spread spectrum communication systemcomprising: means for producing a reference signal; means for producinga message signal having message data; means for combining the referencesignal and the message signal as a combined signal; means fortransmitting the combined signal as a combined spread spectrum signal;means for receiving a carrier signal of the combined signal using thereference signal; and means for recovering the message data usinginformation from the carrier signal, said means for recovering themessage data including means for generating a local reference signal ata second unit and means to synchronize the local reference signal withthe detected reference signal to produce a recovered carrier signal. 14.The system of claim 13 wherein the combined spread spectrum signal is ina code division multiple access format.
 15. The system of claim 13wherein a base station transmits the combined spread spectrum signal anda mobile station receives the combined spread spectrum signal.
 16. Thecommunication system of claim 13, wherein: the means for generating thelocal reference signal at the second unit relays the detected referencesignal, the local reference signal transmitted from the second unit; thesecond unit spread-spectrum processes message data with the localreference signal, and combines the spread-spectrum-processed messagedata with the local reference signal as a CDMA signal; and the secondunit transmits said CDMA signal over a communications channel.
 17. Amethod for use in a spread spectrum communication system, the methodcomprising: producing a reference signal; producing a message signalhaving message data; combining the reference signal and the messagesignal as a combined signal; transmitting the combined signal as acombined spread spectrum signal; receiving the combined spread spectrumsignal; recovering a carrier signal of the combined spread spectrumsignal using the reference signal by generating a local reference signaland synchronizing the local reference signal with the detected referencesignal; and recovering the message data using the recovered carriersignal, thereby demodulating the message signal.
 18. The method of claim17 wherein the combined spread spectrum signal is in a code divisionmultiple access format.
 19. The method of claim 17 wherein a basestation transmits the combined spread spectrum signal and a mobilestation receives the combined spread spectrum signal.
 20. The method ofclaim 17, further comprising: using the generation of the localreference signal to relay the detected reference signal, andtransmitting the local reference signal; processing the local messagedata with the local reference signal, and combining thespread-spectrum-processed local message data with the local-referencesignal as a local-CDMA signal; and transmitting the local-CDMA signalover a communications channel.